Field of the Invention
The present invention relates to a vacuum actuated type booster device adapted mainly for assisting the master cylinder of automobile braking system by the action of vacuum. More particularly, the invention is concerned with an improvement in the vacuum actuated type booster device of a type having a booster shell, a booster piston reciprocatably received by the booster shell, a diaphragm attached at its inner peripheral portion to the rear side of a radially extending disc portion of the booster piston and at its outer peripheral portion to the peripheral wall of the booster shell, a first and a second working chambers defined in the booster shell by the booster piston and the diaphragm, a vacuum source with which the working chambers are communicated and an input rod sidably provided in the booster piston, wherein as the input rod is moved ahead, the second working chamber solely is brought into communication with the atmosphere to impart a forward driving force to the booster piston by the differential pressure acting on the latter.
In the booster device of this type heretofore proposed, the booster piston is formed integrally from an aluminum alloy or the like material. This booster piston has a considerably heavy weight which occupies a comparatively large part of the whole device. The reduction of the weight of booster piston, therefore, contributes greatly to the reduction of the weight of the device as a whole. It is, however, extremely difficult and almost practically impossible to reduce the weight of the booster piston without incurring the degradation of function of the same. In addition, since the booster piston is formed as a unit, the air which has happened to be trapped between the booster piston and the diaphragm cannot escape to the outside so that the durability of the diaphragm is deteriorated and the operation characteristic of the booster piston is adversely affected because the pressure differential between both working chambers is inconveniently absorbed by the trapped air.